Fueling system vapor recovery and containment performance monitor and method of operation thereof

ABSTRACT

A method and apparatus for monitoring and determining fuel vapor recovery performance is disclosed. The dispensing of liquid fuel into a tank by a conventional gas pump nozzle naturally displaces a mixture of air and fuel ullage vapor in the tank. These displaced vapors may be recovered at the dispensing point nozzle by a vapor recovery system. A properly functioning vapor recovery system recovers approximately one unit volume of vapor for every unit volume of dispensed liquid fuel. The ratio of recovered vapor to dispensed fuel is termed the A/L ratio, which should ideally be approximately equal to one (1). The A/L ratio, and thus the proper functioning of the vapor recovery system, may be determined by measuring liquid fuel flow and return vapor flow (using a vapor flow sensor) on a nozzle-by-nozzle basis. The disclosed methods and apparatus provide for the determination of A/L ratios for individual nozzles using a reduced number of vapor flow sensors. The disclosed methods and apparatus also provide for the determination of fuel dispensing system vapor containment integrity, and the differentiation of true vapor recovery failures as opposed to false failures resulting from the refueling of vehicles provided with onboard vapor recovery systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/865,525, filed Oct. 1, 2007, now pending, which is a continuation ofU.S. patent application Ser. No. 11/210,715, filed Aug. 24, 2005, nowU.S. Pat. No. 7,275,417, which is a continuation of U.S. patentapplication Ser. No. 10/935,024, filed Sep. 7, 2004, now U.S. Pat. No.6,964,283, which is a continuation of U.S. patent application Ser. No.10/180,047, filed Jun. 27, 2002, now U.S. Pat. No. 6,802,344, which is adivisional of U.S. patent application Ser. No. 09/725,727, filed Nov.30, 2000, now U.S. Pat. No. 6,622,757, which relates to and claimspriority to U.S. Provisional Patent Application Ser. No. 60/168,029,filed on Nov. 30, 1999, entitled “Fueling System Vapor RecoveryPerformance Monitor,” U.S. Provisional Patent Application Ser. No.60/202,054, filed on May 5, 2000, entitled “Fueling System VaporRecovery Performance Monitor,” and U.S. Provisional Patent ApplicationSer. No. 60/202,659, filed on May 8, 2000, entitled “Method ofDetermining Failure of Fuel Vapor Recovery System.” Each of theforegoing applications is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a vapor recovery performance monitorfor use in connection with gasoline dispensing facilities.

BACKGROUND OF THE INVENTION

Gasoline dispensing facilities (i.e. gasoline stations) often sufferfrom a loss of fuel to the atmosphere due to inadequate vapor collectionduring fuel dispensing activities, excess liquid fuel evaporation in thecontainment tank system, and inadequate reclamation of the vapors duringtanker truck deliveries. Lost vapor is an air pollution problem which ismonitored and regulated by both the federal government and stategovernments. Attempts to minimize losses to the atmosphere have beeneffected by various vapor recovery methods. Such methods include:“Stage-I vapor recovery” where vapors are returned from the undergroundfuel storage tank to the delivery truck; “Stage-II vapor recovery” wherevapors are returned from the refueled vehicle tank to the undergroundstorage tank; vapor processing where the fuel/air vapor mix from theunderground storage tank is received and the vapor is liquefied andreturned as liquid fuel to the underground storage tank; burning excessvapor off and venting the less polluting combustion products to theatmosphere; and other fuel/air mix separation methods.

A “balance” Stage-II Vapor Recovery System (VRS) may make use of adispensing nozzle bellows seal to the vehicle tank filler pipe opening.This seal provides an enclosed space between the vehicle tank and theVRS. During fuel dispensing, the liquid fuel entering the vehicle tankcreates a positive pressure which pushes out the ullage space vaporsthrough the bellows sealed area into the nozzle vapor return port,through the dispensing nozzle and hoe paths, and on into the VRS.

It has been found that even with these measures, substantial amounts ofhydrocarbon vapors are lost to the atmosphere, often due to poorequipment reliability and inadequate maintenance. This is especiallytrue with Stage-II systems. One way to reduce this problem is to providea vapor recovery system monitoring data acquisition and analysis systemto provide notification when the system is not working as required. Suchmonitoring systems may be especially applicable to Stage-II systems.

When working properly, Stage-II vapor recovery results in equalexchanges of air or vapor (A) and liquid (L) between the main fuelstorage tank and the consumer's gas tank. Ideally, Stage-II vaporrecovery produces an A/L ratio very close to 1. In other words, returnedvapor replaces an equal amount of liquid in the main fuel storage tankduring refueling transactions. When the A/L ratio is close to 1,refueling vapors are collected, the ingress of fresh air into thestorage tank is minimized and the accumulation of an excess of positiveor negative pressure in the main fuel storage tank is prevented. Thisminimizes losses at the dispensing nozzle and evaporation and leakage ofexcess vapors from the containment storage tank. Measurement of the A/Lratio thus provides an indication of proper Stage-II vapor collectionoperation. A low ratio means that vapor is not moving properly throughthe dispensing nozzle, hose, or other part of the system back to thestorage tank, possibly due to an obstruction or defective component.

Recently, the California Air Resources Board (CARB) has been producingnew requirements for Enhanced Vapor Recovery (EVR) equipment. Theseinclude stringent vapor recovery system monitoring and In-StationDiagnostics (ISD) requirements to continuously determine whether or notthe systems are working properly. CARB has proposed that, when the A/Lratio drops below a prescribed limit for a single or some sequence offueling transactions, an alarm be issued and the underground storagetank pump be disabled to allow repair to prevent further significantvapor losses. The proposed regulations also specify an elaborate andexpensive monitoring system with many sensors which will be difficult towire to a common data acquisition system.

The CARB proposal requires that Air-to-Liquid (A/L) volume ratio sensorsbe installed at each dispensing hose or fuel dispensing point andpressure sensors be installed to measure the main fuel storage tankvapor space pressure. Note that the term ‘Air’ is used loosely here torefer to the air-vapor mix being returned from the refueled vehicle tankto the Underground storage tank. The sensors would be wired to a commondata acquisition system used for data logging, storage, and limitedpass/fail analysis. It is likely that such sensors would comprise AirFlow Sensors (AFS's).

A first embodiment of the present invention provides a more practicaland less expensive solution than that proposed by CARB, which cansubstantially provide the monitoring capabilities needed. In this firstembodiment of the present invention, the multiple AFS's called for bythe CARB proposal may be replaced by fewer, or only one, AFS inconjunction with a more sophisticated AFS data analysis method.

With respect to use of vapor pressure sensors, CARB also proposes thatthese sensors be used to passively monitor the level of pressure in themain fuel storage tank vapor space, which is common to the fuelingfacility, to not only provide indication of proper operation of Stage-IIvapor recovery methods, but also system containment integrity. This isdone by monitoring the pressure patterns that occur within the storagetank during the various phases of storage tank and dispenser operation.The complexity of these patterns is a function of the type of Stage-IIsystem in use.

CARB has proposed putting constraints on the pressure versus timerelationships to identify when the vapor recovery system is causingundesirably high pressures for long enough time periods. when the vaporrecovery system produces these elevated pressures, it may forcesignificant amounts of vapor past the pressure relief valve at the endof the storage tank vent pipe or out of other leaky system valves andfittings and into the atmosphere as air pollution.

CARB proposes a passive test for identifying elevated storage tankpressures. The purpose of the passive test is to determine whethervapors are being properly retained in the storage tank vapor space. Thisis done by continuously monitoring and watching for evidence of anon-tight or improperly operated vapor recovery components by trackingsmall pressure levels over time and comparing them to prescribedoperating requirements.

For instance, for a vapor recovery system that is intended tocontinuously maintain negative storage tank vapor space pressures, theCARB proposed requirements were (at one time) that an error conditionwould exist when pressure exceeds (i.e. is higher than) −0.1 inch watercolumn (w.c.) for either more than one (1) consecutive hour, or morethan 3 hours in any 24 hour period. An error condition would also existwhen pressure exceeds (i.e. is higher than) +0.25 inches w.c. for eithermore than one (1) consecutive hour, or more than 3 hours in any 24 hourperiod. An error condition would also exist if pressure exceeded +1.0inches w.c. for more than 1 hour in any 24 hour period. Determination ofthe foregoing error conditions requires frequent pressure measurements,data storage, and analysis. CARB has struggled with these requirementsfor a passive-type test and has changed them more than once.

In a second embodiment of the invention the CARB proposed passivepressure monitoring test may be augmented or replaced with an activepressure “tightness” or “leakage” test which provides a more definitiveindication of system containment integrity. The active tightness testmay only need to be run occasionally to find a break in the system. Aonce a day or once a month test is consistent with the intent of thevariously proposed CARB test pass/fail criteria.

In yet another embodiment of the invention, the CARB proposed passivetest for leakage may be replaced with an improved passive test for vaporleakage. Instead of measuring absolute pressure in the vapor containingelements of a facility, in the improved test changes in pressure overtime are used to determine whether vapors are leaking from the system.

Both the aforementioned CARB methods for determining vapor recoverysystem performance and those of the invention may be detrimentallyeffected by the introduction of vehicles with Onboard Refueling VaporRecovery (ORVR) devices that recover refueling vapors onboard thevehicle. Vapors produced as a result of dispensing fuel into an ORVRequipped vehicle are collected onboard, and accordingly, are notavailable to flow through a vapor return passage to an AFS formeasurement. Thus, refueling an ORVR equipped vehicle results in apositive liquid fuel flow reading, but no return vapor flow reading(i.e. an A/L ratio equal to 0 or close thereto)—a condition thatnormally indicates vapor recovery malfunction. Because the vaporrecovery system cannot distinguish between ORVR equipped vehicles andconventional vehicles, the vapor recovery system may be falselydetermined to be malfunctioning when an ORVR equipped vehicle isrefueled.

In the coming years, 2000 to 2020 and beyond, the proportion of ORVRvehicles in use will increase. Therefore this problem will be becomemore severe in the coming decades. If A/L sensing is to be usedsuccessfully for vapor recovery system monitoring, then a method isneeded to distinguish between failed vapor recovery test events causedby an ORVR vapor-blocking vehicle and true failed vapor recovery testevents (which can only occur for non-ORVR equipped vehicles).

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a methodand system for determining acceptable performance of a vapor recoverysystem in a fueling facility.

It is another object of the present invention to provide a method andsystem for measuring the return flow of vapors from a dispensing pointto a main fuel storage tank.

It is yet another object of the present invention to reduce the numberof devices required to determine A/L ratios for individual dispensingpoints in a fueling facility.

It is still yet another object of the present invention to provide amethod and system for determining the integrity of vapor containment ina main fuel storage tank.

It is still a further object of the present invention to provide amethod and system for analyzing and indicating vapor recoveryperformance in a fueling facility.

It is still another object of the present invention to provide a systemand method for determining true vapor recovery system failures.

It is yet another object of the present invention to provide a systemand method for distinguishing between low A/L readings caused by a vaporrecovery system failure and low A/L readings caused by the fueling of anORVR-equipped vehicle.

Additional objects and advantages of the invention are set forth, inpart, in the description which follows, and, in part, will be apparentto one of ordinary skill in the art from the description and/or from thepractice of the invention.

SUMMARY OF THE INVENTION

In response to the foregoing challenges, applicants have developed aninnovative system for monitoring vapor recovery in a liquid fueldispensing facility having at least one fuel dispensing point connectedto a main fuel storage system by a means for supplying liquid fuel tothe dispensing point and a means for returning vapor from the dispensingpoint, said monitoring system comprising: a vapor flow sensoroperatively connected to the means for returning vapor and adapted toindicate the amount of vapor flow through the means for returning vapor;a liquid fuel dispensing meter operatively connected to the means forsupplying liquid fuel and adapted to indicate the amount of liquid fueldispensed at the at least one fuel dispensing point; and a centralelectronic control and diagnostic arrangement having, a means fordetermining a ratio of vapor flow to dispensed liquid fuel for the atleast one fuel dispensing point, said determining means receivingdispensed liquid fuel amount information from the liquid fuel dispensingmeter and receiving vapor flow amount information from the vapor flowsensor, wherein the acceptability of vapor recovery for the fueldispensing point is determined by said ratio of vapor flow to dispensedliquid fuel.

Applicants have also developed an innovative system for monitoring vaporrecovery in a liquid fuel dispensing facility having at least two fueldispensing points connected to a main fuel storage system by a vaporreturn pipeline, said monitoring system comprising: a vapor flow sensoroperatively connected to the vapor return pipeline; means fordetermining dispensed liquid fuel amount information for each fueldispensing point; and a means for determining a ratio of vapor flow todispensed liquid fuel for the fuel dispensing points based on vapor flowsensor readings and dispensed liquid fuel amount information, whereinthe acceptability of vapor recovery for the fuel dispensing points isdetermined by said ratio of vapor flow to dispensed liquid fuel.

Applicants have also developed an innovative method of monitoring vaporrecovery in a liquid fuel dispensing facility having at least one fueldispensing point connected to a main fuel storage system by a means forsupplying liquid fuel to the dispensing point and a means for returningvapors from the dispensing point, said monitoring method comprising thesteps of: determining at multiple times an amount of vapor flow throughthe means for returning vapors; determining at multiple times an amountof liquid fuel dispensed through the means for supplying liquid fuel;and determining a ratio of vapor flow to dispensed liquid fuel for thefuel dispensing point based on the amount of vapor flow through themeans for returning vapors and the amount of liquid fuel dispensedthrough the means for supplying liquid fuel, wherein the acceptabilityof vapor recovery for the fuel dispensing point is determined by saidratio of vapor flow to dispensed liquid fuel.

Applicants have still further developed an innovative system formonitoring vapor containment in a liquid fuel dispensing facility havinga main fuel storage system connected by a vent pipe-pressure reliefvalve arrangement to atmosphere, said monitoring system comprising: apressure sensor operatively connected to the vent pipe; a vaporprocessor operatively connected to the vent pipe; and means fordetermining the acceptability of vapor containment in the main fuelstorage system, said determining means being operatively connected tothe pressure sensor to receive pressure level information therefrom andbeing operatively connected to the vapor processor to selectively causethe vapor processor to draw a negative pressure in the main fuel storagesystem.

Applicants have developed an innovative method of monitoring vaporcontainment in a liquid fuel dispensing facility having at least onemain fuel storage tank connected by a vent pipe-pressure relief valvearrangement to atmosphere, said monitoring method comprising the stepsof: identifying the start of an idle period for the liquid fueldispensing facility; monitoring the liquid fuel dispensing facility toconfirm maintenance of the idle period; determining whether pressure inthe main fuel storage tank is equal or below a minimum level;selectively adjusting pressure in the main fuel storage tank to a presetlower level when the previously determined pressure is above the minimumlevel; monitoring variation of the pressure in the main fuel storagetank during the remainder of the idle period; determining the end of theidle period; and determining the acceptability of vapor containment inthe main fuel storage tank based on the variation of the pressure duringthe idle period.

Applicants also developed an innovative method of determining vaporrecovery system failures associated with a single fuel dispensing point,said method comprising the steps of: determining the vapor flow todispensed fuel ratios for a plurality of fuel dispensing points;determining the number of vapor flow to dispensed fuel ratios that arebelow a preset minimum for each of the plurality of fuel dispensingpoints; determining the average number of vapor flow to dispensed fuelratios below the preset minimum for the plurality of fuel dispensingpoints; and comparing the number vapor flow to dispensed fuel ratiosbelow the preset minimum for each of the plurality of fuel dispensingpoints to the average number of vapor flow to dispensed fuel ratiosbelow the present minimum to determine whether the vapor recovery systemassociated with each of the plurality of fuel dispensing points hasfailed.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention as claimed. The accompanyingdrawings, which are incorporated herein by reference and whichconstitute a part of this specification, illustrate certain embodimentsof the invention, and together with the detailed description serve toexplain the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the followingdrawings in which like reference numerals designate like elements andwherein:

FIG. 1 is a schematic view of a fueling system vapor recoveryperformance monitor in accordance with an embodiment of the presentinvention.

FIG. 2 is a schematic view of a fueling system vapor recoveryperformance monitor in accordance with another embodiment of the presentinvention.

FIG. 3 is a graph used to convert vapor leakage rates based on ullagepressures.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the invention is described in connection with FIG.1, which shows a vapor recovery and containment monitoring system foruse in a liquid fuel dispensing facility 10. The dispensing facility 10may include a station house 100, one or more fuel dispenser units 200, amain fuel storage system 300, means for connecting the dispenser unitsto the main fuel storage system 400, and one or more vapor (or air) flowsensors (AFS's) 500.

The station house 100 may include a central electronic control anddiagnostic arrangement 110 that includes a dispenser controller 120,dispenser current loop interface wiring 130 connecting the dispensercontroller 120 with the dispenser unit(s) 200, and a combined dataacquisition system/in-station diagnostic monitor 140. The dispensercontroller 120 may be electrically connected to the monitor 140 by afirst wiring bus 122. The interface wiring 130 may be electricallyconnected to the monitor 140 by a second wiring bus 132. The monitor 140may include standard computer storage and central processingcapabilities, keyboard input device(s), and audio and visual outputinterfaces among other conventional features.

The fuel dispenser units 200 may be provided in the form of conventional“gas pumps.” Each fuel dispenser unit 200 may include one or more fueldispensing points typically defined by the nozzles 210. The fueldispenser units 200 may include one coaxial vapor/liquid splitter 260,one vapor return passage 220, and one fuel supply passage 230 per nozzle210. The vapor return passages 220 may be joined together beforeconnecting with a common vapor return pipe 410. The units 200 may alsoinclude one liquid fuel dispensing meter 240 per nozzle 210. The liquidfuel dispensing meters 240 may provide dispensed liquid fuel amountinformation to the dispenser controller 120 via the liquid fueldispensing meter interface 270 and interface wiring 130.

The main fuel storage system 300 may include one or more main fuelstorage tanks 310. It is appreciated that the storage tanks 310 maytypically be provided underground, however, underground placement of thetank is not required for application of the invention. It is alsoappreciated that the storage tank 310 shown in FIGS. 1 and 2 mayrepresent a grouping of multiple storage tanks tied together into astorage tank network. Each storage tank 310, or a grouping of storagetanks, may be connected to the atmosphere by a vent pipe 320. The ventpipe 320 may terminate in a pressure relief valve 330. A vapor processor340 may be connected to the vent pipe 320 intermediate of the storagetank 310 and the pressure relief valve 330. A pressure sensor 350 mayalso be operatively connected to the vent pipe 320. Alternately, it maybe connected directly to the storage tank 310 or the vapor return pipe410 below or near to the dispenser 200 since the pressure is normallysubstantially the same at all these points in the vapor containmentsystem. The storage tank 310 may also include an Automatic Tank GaugingSystem (ATGS) 360 used to provide information regarding the fuel levelin the storage tank. The vapor processor 340, the pressure sensor 350,and the automatic tank gauging system 360 may be electrically connectedto the monitor 140 by third, fourth, and fifth wiring busses 342, 352,and 362, respectively. The storage tank 310 may also include a fill pipeand fill tube 370 to provide a means to fill the tank with fuel and asubmersible pump 380 to supply the dispensers 200 with fuel from thestorage tank 310.

The means for connecting the dispenser units and the main fuel storagesystem 400 may include one or more vapor return pipelines 410 and one ormore fuel supply pipelines 420. The vapor return pipelines 410 and thefuel supply pipelines 420 are connected to the vapor return passages 220and fuel supply passages 230, respectively, associated with multiplefuel dispensing points 210. As such, a “vapor return pipeline”designates any return pipeline that carries the return vapor of two ormore vapor return passages 220.

The AFS 500 is operatively connected to a vapor return pipeline 410. Abasic premise of the system 10 is that it includes at most one AFS 500(also referred to more broadly as vapor flow sensors) for each fueldispenser unit 200. Thus, the AFS 500 must be operatively connected tothe vapor return system downstream of the vapor return passages 220. Ifsuch were not the case, the system would include one AFS 500 per nozzle210 which violates the basic premise of the invention. Each AFS 500 maybe electrically connected to the monitor 140 by a sixth wiring bus 502.

In order to determine the acceptability of the performance of vaporrecovery in the facility 10, the ratio of vapor flow to dispensed liquidfuel is determined for each fuel dispensing point 210 included in thefacility. This ratio may be used to determine if the fuel dispensingpoint 210 in question is in fact recovering an equal volume of vapor foreach unit volume of liquid fuel dispensed by the dispensing point 210.

In the embodiment of the invention shown in FIG. 1, each dispensingpoint 210 is served by an AFS 500 that is shared with at least one otherdispensing point 210. Mathematical data processing (described below) isused to determine an approximation of the vapor flow associated witheach dispensing point 210. The amount of fuel dispensed by eachdispensing point 210 is known from the liquid fuel dispensing meter 240associated with each dispensing unit. Amount of fuel (i.e. fuel volume)information may be transmitted from each dispensing meter 240 to thedispenser controller 120 for use by the monitor 140. In an alternativeembodiment of the invention, the dispensing meters 240 may be directlyconnected to the monitor 140 to provide the amount of fuel informationused to determine the A/L ratio for each dispensing point 210.

Each AFS 500 measures multiple (at least two or more) dispensing pointreturn vapor flows. In the embodiment of the invention shown in FIG. 1,a single AFS 500 measures all the dispensing point vapor flows for thefacility 10. In the case of a single AFS per facility 10, the AFS isinstalled in the single common vapor return pipeline which runs betweenall the dispensers as a group, which are all tied together into a commondispenser manifold pipe, and all the main fuel storage tanks as a group,which are all tied together in a common tank manifold pipe. Variousgroupings of combinations of feed dispensing point air flow's per AFSare possible which fall between these two extremes described.

With reference to a second embodiment of the invention shown in FIG. 2,it is appreciated that multiple AFS's 500 could be deployed to measurevarious groupings of dispensing point 210 vapor flows, down to a minimumof only two dispensing point vapor flows. The latter example may berealized by installing one AFS 500 in each dispenser housing 200, whichtypically contains two dispensing point's 210 (one dispensing point perdispenser side) or up to 6 dispensing points (hoses) in Multi-ProductDispensers (MPD's) (3 per side). The vapor flows piped through the vaporreturn passage 220 may be tied together to feed the single AFS 500 inthe dispenser housing.

As stated above, the monitor 140 may connect to the dispenser controller120, directly to the current loop interface wiring 130 or directly tothe liquid fuel dispensing meter 240 to access the liquid fuel flowvolume readings. The monitor 140 may also be connected to each AFS 500at the facility 10 so as to be supplied with vapor flow amount (i.e.vapor volume) information. The liquid fuel flow volume readings areindividualized fuel volume amounts associated with each dispensing point210. The vapor flow volume readings are aggregate amounts resulting fromvarious groupings of dispensing point 210 vapor flows, which thereforerequire mathematical analysis to separate or identify the amountsattributable to the individual dispensing points 210. This analysis maybe accomplished by the monitor 140 which may include processing means.Once the vapor flow information is determined for each dispensing point210, the A/L ratios for each dispensing point may be determined and apass/fail determination may be made for each dispensing point based onthe magnitude of the ratio. It is known that the ratio may vary from 0(bad) to around 1 (good), to a little greater than 1 (which, dependingupon the facility 10 design, can be either good or bad), to much greaterthan 1 (typically bad). This ratio information may be provided to thefacility operator via an audio signal and/or a visual signal through themonitor 140. The ratio information may also result in the automatic shutdown of a dispensing point 210, or a recommendation for dispensing pointshut down.

The embodiments of the invention shown in FIGS. 1 and 2 may provide asignificant improvement over known systems due to the replacement of themultiple AFS's 500 (one per dispensing point, typically anywhere from 10or 12 up to 30 or more per site) and their associated wiring with asingle, or fewer AFS's 500 (about ½ as many or less, depending upondispensing point groupings).

With reference to the embodiments of the invention shown in both FIGS. 1and 2, the mathematical analysis performed in the monitor 140 isdesigned to find correlations between aggregate vapor volume measuredduring AFS 500 ‘busy periods’ and individual dispensing point 210dispensed liquid fuel volume readings. The analysis is done separatelyfor each AFS 500 and it's associated dispensing point group (two or moredispensing point's). The end result is a set of estimated dispensingpoint A/L ratios, one ratio per dispensing point. After a group of AFS500 busy period data records are accumulated, a series of mathematicalsteps accomplish this beginning with a simple, 1-variable functionsolution and ending with more complex function solutions until allratios are determined. If a ratio can be determined in an earlier step,it is not necessary to estimate it in a subsequent step (it can be setas a constant in later steps to simplify computation of any remainingunknown ratios). The sequence of solvable function types are:

Type 1: A single linear function with one unknown for any AFS busyrecords with only 1 active dispensing point.

Type 2: Two linear functions with two unknowns for any pair of similarAFS busy records with 2 (identical) active dispensing point's (twosimultaneous equations with two unknowns).

Type 3: Three or more linear functions each with two or more unknownsfor any remaining (unsolved) set of AFS busy records (at least as manyfunctions as unknowns).

Each AFS 500 busy period data record is formed after the AFS becomesidle by recording the aggregate vapor volume, A, and the individualmetered liquid volumes, L_(m), where the subscript, m, denotes thedispensing point or meter number. This number ranges from 1 to M totalmeters. Idle detection can be done by various means, including:

1) the monitor 140 can track reported dispenser meter 240 start/stopevents from the dispenser controller 120, the dispenser current loopwiring 130, or directly from the liquid fuel dispensing meter 240; or

2) the Automatic Tank Gauging System 360 can provide main fuel storagetank 310 liquid fuel levels to the monitor 140 for detection of staticlevel conditions (no ongoing dispensing) in all the storage tanks 310.

The latter method (No. 2) can be used if it is desired that all AFS's500 be idle prior to forming AFS busy data records. In the case of asingle AFS 500 per facility 10 (shown in FIG. 1), this method can alwaysbe used.

The simple form of the relationship between A, L, and the A/L ratio, R,for an AFS busy record with one (1) active dispensing point is:

A=L_(m)R_(m)

so the simple solution for function type 1 is:

R _(m) =A/L _(m)

where R_(m) is the estimated A/L ratio for active dispensing point(meter), m.

In the more general case, each AFS busy period data record, n, has ameasured aggregate vapor volume, A_(n), and the individual meteredliquid fuel volumes, L_(nm), where the first subscript, n, denotes thedata record number and the second subscript, m, denotes the dispensingpoint or meter number as before. The record number, n, ranges from 1 toN total records.

The generalized form of the relationship between A_(n), L_(nm), andR_(m) for multiple-dispensing point records is:

A _(n) =L _(n1) R ₁ +L _(n2) R ₂ +L _(n3) R ₃ + . . . L _(nm) R _(m)

In the case of a pair of similar busy records with 2 active dispensingpoint's (same 2 dispensing point's in both records) the relationshipsare:

A ₁ =L ₁₁ R ₁ +L ₁₂ R ₂

A ₂ =L ₂₁ R ₁ +L ₂₂ R ₂

so the solutions for functions of type 2 are:

R ₁=(A ₁ L ₂₂ −A ₂ L ₁₂)/(L ₁₁ L ₂₂ −L ₁₂ L ₂₁)

R ₂=(A ₂ L ₁₁ −A ₁ L ₂₁)/(L ₁₁ L ₂₂ −L ₁₂ L ₂₁)

Functions of type 3 can be solved as a least squares problem usingstandard matrix arithmetic.

Example record data set with subscript notation:

n A_(n) L_(n1) L_(n2) L_(n3) etc . . . L_(nM) 1 18 0 12 6 etc . . . 0 233 10 15 0 etc . . . 8 3 21 7 0 0 etc . . . 14 etc . . . N 18 0 0 18 etc. . . 0

For the entire data set, the matrix relationship is:

$\begin{bmatrix}A_{1} \\A_{2} \\A_{3} \\\vdots \\A_{n}\end{bmatrix} = {\begin{bmatrix}L_{11} & L_{12} & \ldots & L_{1\; m} \\L_{21} & L_{22} & \ldots & L_{2\; m} \\L_{31} & L_{32} & \ldots & L_{3\; m} \\\vdots & \vdots & \vdots & \vdots \\L_{n\; 1} & L_{n\; 2} & \ldots & L_{n\; m}\end{bmatrix}\begin{bmatrix}{R\; 1} \\{R\; 2} \\\vdots \\R_{m}\end{bmatrix}}$ or A = LR

The solution for the ratio vector, R, is:

R=(L ^(T) L)⁻¹ L ^(T) A

where the first term is the inverse of the transposed n×m matrix, L,times itself which results in an m x m matrix, the middle term is thetransposed matrix, L, which is an m×n matrix, and the last term is thevector A of length n, all of which results in the vector R, of length m(one A/L ratio per meter).

This approach can provide good estimates of the true A/L ratios, evenwith excessive variability (noise) in the sensor readings. More recordsresult in better estimates for a given level of variability but theremust be at least as many records as unknowns for minimal performance.

Dispensing point ratio solutions are based on the simplest function typepossible. As a data set is processed and ratio solutions are determined,they are in turn used to simplify solutions for remaining records in anyrecord set. As an example, if two records exist in a set, one of type 1(a single active dispensing point busy period), and a second with twoactive dispensing points, one of which is the same dispensing point asin the first record, the first record is solved directly as a type 1function and it's ratio result is used to simplify the function for thesecond record. This produces a second type 1 function.

Example records (2):

n A_(n) L_(n1) L_(n2) 1 5 — 10 2 19.5 12 15

Initial functions:

A₁=L₁₂R₂

5=10R₂

A ₂ =L ₂₁ R ₁ +L ₂₂R₂

19.5=12R ₁+15R ₂

Solve first, substitute solution in second to simplify:

5=10R ₂

R ₂=5/10=0.5

19.5=12R ₁+15 R ₂

19.5+12R ₁+15*0.5=12R ₁+7.5

Solve second as a type 1 function:

19.5=12R ₁+7.5

12=12R ₁

R ₁=12/12=1.0

This simplification method is used at each step of the data set solutionprocess:

Step 1: Form simple (1-dispensing point) or generalized function formsfor each record.

Step 2: Solve all Type 1 functions.

Step 3: Substitute solutions from prior step into remaining set offunctions.

Step 4: Reduce all functions to simpler forms and repeat from step 2.

Step 5: Find and solve any Type 2 function pairs.

Step 6: Substitute solutions from prior step into remaining set offunctions.

Step 7: Reduce all functions to simpler forms and repeat from step 2.

Step 8: If possible, solve remaining functions as a Type 3 least squaresproblem.

Step 9: If step 8 is not possible, wait for more data records to solvethe remaining functions.

Alternatively, replace the 9-step sequence with steps 8 and 9 alone.This approach has the benefit of always averaging or reducing theeffects of variability in the sensor readings.

The various embodiments of the invention discussed herein may also beused to detect vapor recovery equipment failures. Stage-II vaporrecovery equipment failures can have two distinct effects on patterns ofA/L ratios. The failures are determined by identifying these patterns inthe solved ratio set. The first type of failure involves a dispensingpoint nozzle 210, a hose 212, or vapor return passage 220 pathrestriction, or a vacuum assist pump failure which blocks or reducesair-vapor flow. The above solution methods may be used to identify thistype of failure by identification of one dispensing point with aconsistently lowered ratio.

The second type of failure that can occur involves a dispensing point210 with a defective air valve which does not close properly to blockreverse vapor flow (i.e. out of the nozzle) when the dispensing point isidle. In such a case the ratio for the defective dispensing point willnot be affected because when the dispensing point is active, the vaporflow is normal. However, when idle, vapors from other active dispensingpoints can be pushed past the defective air valve, out of the leakydispensing point nozzle, and into the atmosphere. The active dispensingpoint(s) AFS 500 may or may not register the amount of lost vapor,depending upon whether the leaking dispensing point is part of the AFSgroup (won't register) or not (will register). If not, the idle AFS 500will register reverse vapor flow. In that case, the leaking dispensingpoint can be detected by the reverse flow signal when it should be idle.

Using the above solution methods described in connection with the firstand second embodiments of the invention, when the leaking dispensingpoint is a member of the active AFS 500 group it results in loweredratios for all dispensing points in the group except for the leakingdispensing point. Also, the lowered ratios vary depending upon thenumber of active dispensing point's during each busy period. When more(good) dispensing point's are active in an AFS 500 group, the lost vaporeffect is shared in the solution, resulting in less depression of theindividual ratios. Furthermore, if only part of the vapors escape to theatmosphere, the effect is reduced, resulting in less depression of theindividual ratios. Accordingly, a post-solution analysis may beconducted on the ratio patterns to determine the likely failure type,active dispensing point restriction or idle dispensing point leak.

A third embodiment of the invention concerns the use of a single vaporpressure sensor 350 (same as CARB requirement) to actively determine thetightness of the overall vapor containing elements of the facilityincluding the fuel storage system 300, (which includes the vent pipe320, pressure relief valve 330, etc.), the vapor return pipelines 410,the vapor/liquid splitter 260, the vapor return passages 220, thedispenser hose 212, the nozzle 210, etc. The vapor pressure sensor 350may be connected anywhere in the fuel storage system 300 or the pipelinesystem 400, which includes but is not limited to the storage tank 310vapor-space, the common vapor return pipeline 410, or the storage tankvent pipe 320. The vapor pressure sensor 350 may be used periodically toactively measure the leakage of vapors from the overall system insteadof constantly measuring for leakage amount.

The method in accordance with the third embodiment of the invention maybe carried out as follows. The monitor 140 may be connected to andaccess pressure readings from the vapor pressure sensor 350. The monitor140 controls the active test which is initiated by determining an idleperiod during which none of the dispensing units 200 are in operation(similar to the A/L detection method using either dispensing meterevents or ATGS tank levels). The idle condition may be continuouslymonitored and the test aborted if any dispensing units go into operationduring the test. During the idle period the vapor pressure sensor 350 isused to determine the pressure in the system (i.e. the pressure in thestorage tank 310). If the pressure is not adequately negative (vacuum)for the test, the vapor processor 340 may be turned on to draw anegative pressure in the storage tank 310 as it processes vapors. If thevapor processor 340 is used, the monitor 140 may be used to monitor thevapor pressure readings until they become adequately negative, typically−2 or −3 inches w.c. Once the vapor pressure is adequately negative, thevapor processor 340 may be turned off. Thereafter the vapor pressuresensor 350 readings may be monitored during the remaining idle time. Ifthe system is adequately tight, the negative pressure readings shouldhold or degrade only slowly. If the negative pressure degrades toorapidly toward zero, the monitor 140 may indicate that the system hasfailed the leakage test. A pass/fail threshold is used to make thisdetermination. It can be set as a percentage of the initial negativepressure amount based on the desired detection sensitivity and should berelated to the amount of air inflow detected relative to total storagetank 310 vapor space (uliage volume).

In an alternative of the third embodiment of the invention, a single ormultiple AFS's 500 located in the common or multiple vapor returnpipeline(s) (same as A/L detection equipment) may be included to conductan improved active test for system tightness. While a pressure sensor350 alone suffices for conducting a tightness test, AFS 500 readings canadd to the amount of information available to augment test sensitivityand confirm the tightness condition or help locate the source of a leak.Any air inflow from a leak point will register as flow on the AFS(s)500. Flow and flow direction are a general indicator of the location ofthe source of incoming air (which dispensers and/or tanks/vents). Notethat the AFS 500 readings are generally the more sensitive indicator ofvapor recovery system tightness failure since negative pressuredegradation is small due to the small amount of air inflow over secondsor minutes of time relative to the generally large storage tankvapor-space volumes. For significant negative pressure degradation, theamount of air inflow needs to be a significant portion of the storagetank vapor-space volume which can be in the thousands or tens ofthousands of gallons.

The optional AFS(s) 500, and dispenser controller 120, dispenser currentloop 130, or optional ATGS 360 are connected to the monitor 140 whichacquires and processes the data from the devices to conduct thetightness test and also controls (on/off) the vapor processor 340. Notethat only one vapor pressure sensor 350 is needed for multiple storagetanks 310 as long as they share a common vapor recovery system so thattheir vapor spaces are connected (piped) together.

In another alternative embodiment of the invention, the ATGS 360 may notbe required to conduct an active test for system tightness. In thiscase, the idle state of the vapor recovery system during which thetightness test is conducted must be determined by (lack of) fuelingmeter 240 activity and a precise estimation of leak rate is not possiblesince tank 310 vapor ullage space volume is not known. Instead, ageneral pass/fail indication can be provided when the pressure decays ata preset rate during a test period.

In yet another embodiment of the present invention, the systems shown inFIGS. 1 and 2 may be used to conduct an improved passive vaporcontainment test. This test uses pressure in the vapor containingelements of the facility, barometric pressure, and ullage spacemeasurements to calculate the change in pressure over time for the vaporcontaining elements of the facility. This calculation, which is notusually based on data collected when the facility is operating at −2 to−3 inches w.c., may then be normalized to indicate leakage rates for afacility held at −2 to −3 inches w.c.

This passive method may be initiated by monitoring the pressure of themain fuel storage system 300 or any vapor containing element of thefacility 10 between fuel dispensing periods with the pressure sensor350. Pressure data derived from sequential groupings of monitoredpressures and ullage determinations derived from the ATGS 360 readingsare recorded at periodic intervals by monitor 140. The derived recordeddata permits the determination of rate of change of pressure, p_(rate),versus time, t, obtained from a linear regression model:

p=p _(rate) ·t

within each interval, the main storage system 300 total ullage volume,V_(ullage) represented by the sum of the individual storage tank 310ullage volumes, V_(ullage):

V _(ullage) =v _(ullage1) +v _(ullage2) + . . . +v _(ullageN) for tanks1 to N

where v_(ullage)=(tank capacity)−(volume of fuel in tank)obtained from the ATGS 360, and the average pressure, p_(avg) over eachinterval:

p _(avg)=(p ₁ +p ₂ + . . . +p _(N))/N for pressure samples 1 to N in theinterval

are recorded if the correlation to the linear model is acceptable,generally based on high correlation between pressure with respect totime and the model.

Upon collection of a daily sample of such records, the product ofpressure rate and the total ullage volume, p_(rate)·V_(ullage), issorted by the associated average pressure, p_(avg), and grouped intoequally spaced average pressure ranges. A collection of averages of theproducts, (p_(rate)V_(ullage))_(avg), within each group:

(p _(rate) V _(ullage))avg=((p _(rate) ·V _(ullage))₁+(p _(rate) ·V_(ullage))₂+ . . . +(p _(rate) ·V _(ullage))_(N))/N

for products 1 to N in each group. The midpoints of the average pressureranges, p_(mid), within each group are used with a linear regressionmodel to estimate the rate of change of pressure times ullage volume,P_(rate)V at a selected test pressure, p_(test), of, say, 2 inches ofwater column, if the correlation to the linear model:

P_(rate)V =(P_(rate)V)_(slope) p_(test)

is acceptable generally based on high correlation between the averageproducts, (p_(rate)V_(ullage))_(avg), with respect to midpointpressures, p_(mid), and the model. A typical graph of this model for atank system is shown in FIG. 3. It is noted that the curve must crossthe origin which indicates no rate of change of pressure, thus noleakage, when there is zero pressure drop across any leakage path, sincefor leakage to occur a pressure driving force is needed regardless ofullage volume.

The regression yields the slope coefficient, (P_(rate)V)_(slope), whichis used to calculate the estimated pressure times ullage volume,P_(rate)V at a selected test pressure, p_(test), of, say, 2 inches ofwater column at which a leakage test failure rate can be defined,similar to the standard CARB TP-201.3 test procedure. In other words, ifthere is a leakage path and if the pressure in the ullage space of thetank system is set to 2″ wcg (water column gauge) (above ambientpressure), the tank will leak at the estimated rate, v_(rate), of:

v _(rate) =P _(rate) V(p _(test))/p

where p is the absolute pressure in the tank ullage space, typically410″ wca (water column absolute) (assuming ambient is 408″ wca). Thiscan be interpreted to mean that the rate of volume vapor loss from aleaking tank is equal to the proportional rate of change of absolutepressure times the total ullage volume. Note that p_(test) is a gaugepressure (referenced to ambient) and p is an absolute pressure(referenced to a vacuum). This relationship is derived from the idealgas law, which governs the relationship between pressure, p, and volume,v, in an enclosed space at low pressures and temperatures:

p·v=n·R·T

where n is moles of gas, R is the universal gas constant, and T isabsolute temperature. Replacing n with mass per molecular weight (MW):

p·v=m·R·T/MW

Rearranging terms and replacing constant terms with k:

m=k·p where k=v·MW/(R·T)

Rate of mass loss due to a leak from an enclosed space is found byforming the relationship of the difference between the ending andstarting mass divided by starting mass and the time period of the loss:

(m2−m1)/m1·t=(k·p2−k·p1)/k·p1·t

Δm/(m1·t)=(p2−p1)/p1·t

Δm/(m1·t)=Δp/p1·t

Δm/t=Δp·m1/p1·t

The last form describes the rate of mass loss as a function of startingmass times proportional pressure change rate over the test period. Tofind volume loss rate, relate mass and volume by mass density, ρ:

ρ=m/v or m=ρ·v so m1=ρ1·v and mass loss: Δm=ρ·Δv

Substituting in above equation:

ρ·Δv/t=Δp·ρ1·v/p1·t

Assuming mass density does not change appreciably:

Δv/t=Δp·v/p1·t where ρ1≈ρ

Dropping the subscript and using notation for volume loss rate,v_(rate):

v _(rate) =Δp·v/p·t

which can be interpreted to mean that the volume loss rate is theproportional change of pressure times volume per unit time. But part ofthis expression is the calculated value derived from measurements in theabove section:

v _(rate) =P _(rate) V/p where v _(rate) =Δp·v/t at the selected testpressure, 2″ wcg

Using the above example, the volume leak rate, v_(rate), is:

v _(rate) =P _(rate) V/p=6000/410=14.6 CFH or cubic feet per hour at 2″wcg

As described above, in yet another embodiment of the invention, thesystem may also perform a method of distinguishing between true vaporrecovery failure events and ORVR equipped vehicle refueling events.Identifying a false vapor recovery system failure due to refueling anORVR-equipped vehicle may be accomplished by applying standardstatistical concepts to a group of dispensing or refueling events fromall the dispensing points 210 at a dispensing facility 10 to identifytrue failed vapor collection dispensing points as opposed to failedtests due to ORVR vapor-blocking activity.

There are two assumptions that may be made as a predicate to determiningtrue failed vapor collection: (1) that ORVR and non-ORVR activity occurssomewhat randomly amongst all the dispensing points; and (2) thataverage ORVR activity does not reach 100% of all refueling events (amaximum of 80% can be assumed). Given these assumptions, a group ofvapor collection event A/L measurements taken from all the dispensingpoints 210 at a dispensing facility 10 may be used to make the followingdeterminations:

1) Determine if the proportions of failed (close to zero A/L) andnon-failed events are statistically different at individual dispensingpoints relative to their expected proportions, due to the activity ofORVR vehicles, derived from all the dispensing points; and

2) Determine if the proportion of the failed (close to zero) events ateach dispensing point are statistically different from the proportion ofthe failed events derived from all the dispensing points, which arelargely due to the effect of ORVR vehicles.

As a result of these determinations, the A/L ratio measurements may beused to test for blockage or leakage caused vapor recovery failure, witha mix of ORVR and non-ORVR vehicle activity.

On a regular (e.g. daily) basis, each dispensing point 210 may have anumber of A/L determinations associated with it. It is presumed thatthere are k dispensing points 210 and the i^(th) dispensing point hasn_(i) A/L ratio determinations. Let X_(i) be the number of A/Ldeterminations for dispensing point I that indicate a “zero” or“blocked” A/L ratio. The assumption is that fueling an ORVR vehicle willresult in a zero or blocked A/L ratio. The total number of A/Ldeterminations for the site is:

n=Σn_(i)

and the total number of zero A/L ratios is:

X=ΣX_(i)

An overall test can be conducted to determine whether there are anysignificant differences in the proportion of A/L ratios indicatingblocked vapor flow among the dispensing points 210. This can beaccomplished using a chi-squared test on the table of data from the kdispensers:

Dispenser 1 Dispenser 2 Dispenser k Total Number X1 X2 . . . Xk Xblocked Not blocked n1-X1 n2-X2 . . . nk-Xk N-X Number n1 n2 . . . nk NThe chi-squared statistic is given

by: X ²=Σ(O _(i) −E _(i))² /E _(i)

where O_(i) is the number observed in each cell of the table and E_(i)is the expected number in that cell. The data in the cells indicate thenumber of A/L ratios that indicate a “blocked” condition for eachdispensing point and the number of A/L ratios indicating a “not blocked”condition for that dispenser. The expected number “blocked” ratios fordispenser I is:

E _(i1) =n _(i)(X/N)

and the expected number of “not blocked” ratios for dispenser I is:

n_(i)−E_(i)

The summation is carried out over 2k cells. This statistic is comparedto the critical value from a chi-squared table with k−1 degrees offreedom. If it is significant, there is evidence that the dispensershave different proportions of blocked A/L ratios, so that one or morewould appear to be blocked on at least an intermittent basis.

In turn, an individual test can be performed for each dispenser. Thistests whether each dispenser has a proportion of zero A/L ratios thatexceeds the overall proportion for the station. The following equationmay be used to compute the overall proportion of zero A/L ratios for theperiod:

P=X/N

The following equation may be used to compute the proportion of zero A/Lratios for each dispenser:

p _(i) =x _(i) /n _(i)

From the foregoing calculations, it may be concluded that there isevidence that dispenser I is blocked if:

p _(i) >P+z _(α)(0.16/n _(i))^(1/2)

where z_(α) is the upper a percentage point from a standard normaldistribution. If a 1% significance level is desired, z_(α) is 2.326, forexample, (or 1.645 for a 5% significance level). The number 0.16 in theformula results from assumption of the most conservative case; that 80%of the vehicles are ORVR vehicles. Once a truly blocked dispensing pointis detected, an audio or visual signal may be provided by the monitor140 to indicate this condition. Truly blocked dispensing points may alsobe automatically shut down as a result of such detection.

It will be apparent to those skilled in the art that variousmodifications and variations may be made in the preparation andconfiguration of the present invention without departing from the scopeand spirit of the present invention. For example, various combinationsof the methods described above may be implemented without implementingthe full system shown FIGS. 1 and/or 2. Thus, it is intended that thepresent invention cover the modifications and variations of theinvention.

1. A system for monitoring vapor containment in a liquid fuel dispensingfacility having a fuel storage system that provides fuel for dispensingby at least one fuel dispensing unit, comprising: a pressure sensoroperatively connected to the fuel storage system to measure the pressurein the fuel storage system; and a monitor operatively connected to thepressure sensor wherein the monitor is adapted to: detect if all of theat least one fuel dispensing units are idle, and if all of the at leastone fuel dispensing units are idle: monitor a pressure level in the fuelstorage system over a period of time to determine if the fuel storagesystem contains a leak; and abort the monitoring of the pressure levelin the fuel storage system at any time that any of the at least one fueldispensing units are not idle before the period of time expires.
 2. Thesystem of claim 1, wherein the pressure sensor is located in a locationcomprised from the group consisting of a fuel storage tank, a fuelstorage tank vapor space, a fuel storage tank vent pipe, a pipelinesystem, and a common vapor return pipeline.
 3. The system of claim 1,wherein the monitor detects if the fuel storage system is adequatelytight by monitoring the decay of the pressure level over the period oftime.
 4. The system of claim 3, wherein the monitor detects if the decayof the pressure level over the period of time exceeds a threshold level.5. The system of claim 4, wherein the monitor indicates a fail conditionif the decay of the pressure level over the period of time exceeds thethreshold level.
 6. The system of claim 4, wherein the monitor indicatesa pass condition if the decay of the pressure level over the period oftime does not exceed the threshold level.
 7. The system of claim 1,further comprising a tank gauge that is communicatively coupled to themonitor, wherein the tank gauge provides the liquid fuel levels of thefuel storage system to the monitor.
 8. The system of claim 1, whereinthe monitor is further adapted to receive start and stop eventscommunicated to the at least one fuel dispensing unit to detect if allof the at least one fuel dispensing units are idle.
 9. The system ofclaim 8, wherein the monitor is coupled to either a meter in the atleast one fuel dispensing unit, a dispenser controller coupled to the atleast one fuel dispensing unit, or a dispenser current loop wiringcoupled to the at least one fuel dispensing unit to receive the startand stop events.
 10. The system of claim 1, wherein the monitor reportsthe pressure level to a dispenser controller which is communicativelycoupled to the at least one fuel dispensing unit.
 11. A method ofmonitoring vapor containment in a liquid fuel dispensing facility havinga fuel storage system that provides fuel for dispensing by least onefuel dispensing unit, comprising the steps of: measuring a pressurelevel in the fuel storage system using a pressure sensor operativelyconnected to the fuel storage system; detecting if all of the at leastone fuel dispensing units are idle, and if all of the at least one fueldispensing units are idle: monitoring the pressure level in the fuelstorage system over a period of time to detect if the fuel storagesystem contains a leak; and aborting the monitoring of the pressurelevel in the fuel storage system at any time that any of the at leastone fuel dispensing units are not idle before the period of timeexpires.
 12. The method of claim 11, further comprising determining ifthe fuel storage system is adequately tight by monitoring the decay ofthe pressure level over the period of time.
 13. The method of claim 12,further comprising detecting if the decay of the pressure level over theperiod of time exceeds a threshold level.
 14. The method of claim 13,further comprising indicating a fail condition if the decay of thepressure level over the period of time exceeds the threshold level. 15.The method of claim 13, further comprising indicating a pass conditionif the decay of the pressure level over the period of time does notexceed the threshold level.
 16. The method of claim 11, furthercomprising measuring air flow from the fuel storage system using an airflow sensor coupled to the fuel storage system and detecting if the fuelstorage system contains a leak based on measurements from the air flowsensor.
 17. The method of claim 11 further comprising detecting if allof the at least one fuel dispensing units are idle by detecting a staticlevel condition in liquid fuel levels of the fuel storage system. 18.The method of claim 11, further comprising: receiving start and stopevents communicated to the at least one fuel dispensing unit; anddetecting if all of the at least one fuel dispensing units are idlebased on the start and stop events.
 19. The method of claim 18, furthercomprising receiving the start and stop events from a meter in the atleast one fuel dispensing unit, a dispenser controller coupled to the atleast one fuel dispensing unit, or a dispenser current loop wiringcoupled to the at least one fuel dispensing unit to receive the startand stop events.
 20. The method of claim 11, further comprisingreporting the pressure level to a dispenser controller which iscommunicatively coupled to the at least one fuel dispensing unit. 21.The method of claim 11, further comprising determining a model toestimate a rate of change of pressure times ullage volume of a fuelstorage tank in the fuel storage system for a given test pressure by:calculating a plurality of pressure rate and ullage volume measurementpairs each comprising an average pressure rate in the fuel storage tankusing the pressure sensor, and ullage volume of the fuel storage tank,for a plurality of intervals; calculating pressure mid points from themeasurement pairs; and using the measurement pairs and pressure midpoints with linear regression to create a model of the rate of change ofpressure times ullage volume at selected test pressures.
 22. The methodof claim 21, further comprising calculating a leak rate for a selectedtest pressure by dividing the rate of change of pressure times ullagevolume from the model at the selected test pressure by the selected testpressure.
 23. The method of claim 22, further comprising detecting if aleak exists in the fuel storage tank by determining if the leak rate isgreater than a threshold leak rate.